GnRH Agonists

ABSTRACT

Peptides for promoting the spawning of fish having the formula: 
     
         pGlu--His--Trp--Ser--Tyr--R.sub.6 --Trp--Leu--Pro--R.sub.10 --NHR 
    
     or a nontoxic salt thereof, wherein R 6  is a D-isomer of an α-amino acid selected from the group consisting of 
     Trp, Ala, Phe, Lys, Pro, Met, Leu, Glu, Asn, Arg, Tyr, Cys, His, Chg, Nva, Orn, Thr, Abu, Phg, Ile, Glu, Asp, Nle and Val and wherein R 10  is Gly or des-R 10  and R is H or C n  Y 2n  CY 3 , with Y being H or F and n being 0, 1, 2 or 3, provided that when R 10  is des-R 10 , n is not 0. The D-isomer α-amino acids may contain known substitutions.

This invention was made with Government support under Grant HD-13527 awarded by DHHS(NIH). The Government has certain rights in this invention.

The present invention relates to peptides which influence the release of gonadotropins by the pituitary gland in fish. More particularly, the present invention is directed to a peptide which when administered to fish exhibits potency in releasing gonadotropins and in causing spawning.

BACKGROUND OF THE INVENTION

The pituitary gland is attached to a stalk to the region in the base of the brain known as the hypothalamus and has two principal lobes, the anterior lobe and the posterior lobe. The posterior lobe of the pituitary gland stores and passes onto the general circulation system two hormones manufactured in the hypothalamus, i.e., vasopressin and oxytocin. The anterior lobe of the pituitary gland secretes a number of hormones, which are complex protein or glycoprotein molecules, that travel through the bloodstream to various organs and which, in turn, stimulate the secretion into the bloodstream of other hormones from the peripheral organs. In particular, follicle stimulating hormone (FSH) and luteinizing hormone (LH), sometimes referred to as gonadotropins or gonadotropic hormones, are released by the pituitary gland. These hormones, in combination, regulate the functioning of the gonads to produce testosterone in the testes and progesterone and estrogen in the ovaries, and also regulate the production and maturation of gametes.

The release of a hormone by the anterior lobe of the pituitary gland usually requires a prior release of another class of hormones produced by the hypothalamus. Such a hypothalamic hormone acts as a factor that triggers the release of the gonadotropic hormones, particularly LH. The particular hypothalamic hormone which acts as a releasing factor for the gonadotropins LH and FSH is referred to herein as GnRH, wherein RH stands for "releasing hormone" and Gn signifies that gonadotropin hormones are being released. It is sometimes also referred to as LRF and LHRH. GnRH from mammals, e.g. humans, sheep, pigs, cows, rats, etc., has been previously isolated, identified and synthesized.

Mammalian GnRH has been characterized as a decapeptide having the following structure:

    pGlu--His--Trp--Ser--Tyr--Gly--Leu--Arg--Pro--Gly--NH.sub.2.

Peptides are compounds which contain two or more amino acids in which the carboxyl group of one acid is linked to the amino group of the other acid. The formula for GnRH, as represented above, is in accordance with conventional representation of peptides where the amino group appears to the left and the carboxyl group to the right. The position of the amino groups is identified by numbering the amino groups from left to right. In the case of GnRH, the hydroxyl portion of the carboxyl group at the right-hand end has been replaced with an amino group (NH₂) to give an amide function. The abbreviations for the individual amino acid groups above are conventional and are based on the trivial name of the amino acid: where pGlu is pyroglutamic acid, His is histidine, Trp is tryptophan, Ser is serine, Tyr is tyrosine, Gly is glycine, Leu is leucine, Arg is arginine and Pro is proline. Except for glycine, amino acid residues in the peptides of the invention should be understood to be of the L-configuration unless noted otherwise.

The following abbreviations are used in this application in addition to the usual abbreviations for the trivial names of the more common α-amino acids:

NVa=norvaline

Orn=ornithine

Ile=isoleucine

Nle=norleucine

Nal=β-naphthyl-Ala

Phg=C-phenylglycine

Abu=2-aminobutyric acid

Chg=2-cyclohexyl Gly

OMe=methylester

OBzl=benzyl ester

tBu=tertiary butyl

BOC=tert-butyloxycarbonyl

SUMMARY OF THE INVENTION

The present invention provides peptides which exhibit a high potency to cause the release of gonadotropins in fish. The present invention provides peptides which have a potent effect on the reproduction processes of fish. They are decapeptides and nonapeptides that are analogs of GnRH which have high potency in promoting spawning in fish and may have particular effects upon mammalian reproductive systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, the following peptides are provided which are potent to release gonadotropins in fish and which have the following formula:

    pGlu--His--Trp--Ser--Tyr--R.sub.6 --Trp--Leu--Pro--R.sub.10 --NHR

wherein R₆ is a D-isomer of an α-amino acid selected from the group consisting of

Trp, Ala, Phe, Lys, Pro, Met, Leu, Glu, Asn, Arg, Tyr, Cys, His, Chg, Nva, Orn, Thr, Abu, Phg, Ile, Glu, Asp, Nle and Val and wherein R₁₀ is Gly or des-R₁₀ and R is H or C_(n) Y_(2n) CY₃ with Y being H or F and n being 0, 1, 2 or 3, provided that when R₁₀ is des-R₁₀, n is not 0.

The above-identified D-isomers can be substituted with known substituents without detracting from the potency, and in some instances the substituted residues may somewhat increase potency. Accordingly, the substituted residues are considered to be equivalents of the unsubstituted residues enumerated above for purposes of this application. Examples of substitutions are set forth as follows: His can be substituted with benzyl or the like on the imidazole nitrogen; Val can be substituted with 3-methyl, 3-tBu, thio, 3-methylthio or the like; Glu can be substituted with OtBu or the like; Asp can be substituted with OtBu, anilide or the like; Ala can be substituted with thienyl, 1-naphthyl, 2-naphthyl, perhydronaphthyl, 2-anthryl, 2-fluorenyl, 3-phenanthryl, 4-biphenylyl, perhydrobiphenyl, 2,2-diphenylmethyl, 1-adamantyl, trimethylcyclohexyl, tributylcyclohexyl or the like; Glu can have its amino group substituted by ethyl, cyclohexyl, benzyl, adamantyl or the like; Phe can be substituted with trimethyl, pentamethyl, NO₂, Cl, OCH₃, CH₃, AcNH, NH₂, dichloro, pentafluoro, tri(n-butyl), trimethylcyclohexyl or the like; Orn can be substituted with lauroyl or the like; Thr can be substituted with OtBu or the like; Ser can be substituted with phenyl, OtBu, OBzl or the like; Tyr can be substituted with OMe or the like; Lys can be substituted with benzoyl, BOC, terephthaloyl, indomethacinyl, lauryl, stearoyl, succinoyl, isophthaloyl, acetylsaliciloyl, tricarballylic acid, trimesoyl, phenyl, chlorambucil or the like on the epsilon-carbon; Trp can be substituted with O-nitrophenylsulfenyl or the like; and Cys can be substituted with tBu, acetamidomethyl or the like.

Examples of preferred substituted amides for the C-terminal include ethyl, propyl, 2,2,2-trifluoroethyl, 2,2-difluoroethyl, pentafluoroethyl, trifluoromethyl and 3,3,3,2,2-pentafluoropropyl.

The peptides are synthesized by a suitable method, such as by exclusively solid-phase techniques, by partial solid-phase techniques, by fragment condensation, by classical solution addition or by the employment of recently developed recombinant DNA techniques. For example, the techniques of exclusively solid-state synthesis are set forth in the textbook "Solid-Phase Peptide Synthesis," Stewart and Young, Freeman and Co., San Francisco, 1969, and are exemplified by the disclosure of U.S. Pat. No. 4,105,603, issued Aug. 8, 1978 to Vale et al. The fragment condensation method of synthesis is exemplified in U.S. Pat. No. 3,972,859 (Aug. 3, 1976). Other available syntheses are exemplified by U.S. Pat. No. 3,842,067 (Oct. 15, 1974) and U.S. Pat. No. 3,862,925 (Jan. 28, 1975).

Common to such syntheses is the protection of the liable side chain groups of the various amino acid moieties with suitable protecting groups which will prevent a chemical reaction from occurring at that site until the group is ultimately removed. Usually also common is the protection of the alpha-amino group on an amino acid or a fragment while that entity which is being added reacts at the carboxyl group, followed by the selective removal of the alpha-amino protecting group to allow subsequent reaction to take place at that location.

Among the known classes of α-amino protecting groups are (1) acyl-type protecting groups, such as formyl, trifluoroacetyl, phthalyl, Tos, benzoyl, benzensulfonyl, nitrophenylsulfenyl, tritylsulfenyl, o-nitrophenoxyacetyl, acrylyl, chloroacetyl, acetyl and γ-chlorobutyryl; (2) aromatic urethan-type protecting groups, e.g., benzyloxycarbonyl (Z) and substituted benzyloxycarbonyl, such as p-chloro-benzyloxycarbonyl, p-nitrobenzyloxycarbonyl, p-bromobenzyl-oxycarbonyl and p-methoxybenzyloxycarbonyl; (3) aliphatic urethan protecting groups, such as terbutyloxycarbonyl (Boc), diisopropylmethoxycarbonyl, isopropyloxycarbonyl, ethoxycarbonyl and allyloxycarbonyl; (4) cycloalkyl urethan-type protecting groups, such as cyclopentyloxycarbonyl, adamantyloxycarbonyl and cyclohexyloxycarbonyl; (5) thiourethan-type protecting groups, such as phenylthiocarbonyl; (6) alkyl-type protecting groups, such as allyl, triphenylmethyl(trityl) and benzyl; (7) trialkylsilane groups, such as trimethylsilane. The preferred -amino protecting group is Boc.

The peptides of the present invention are preferably synthesized by a solid phase technique using a chloromethylated resin, a methylbenzhydrylamine resin (MBHA) or a benzhydrylamine (BHA) resin. A benzyl-polyamide polymer is commercially available and is discussed in detail in Bioorganic Chemistry, 8, 351-370 (1979) where a preferred version of it is discussed in connection with the synthesis illustrated in FIG. 6. When it is employed, the side-chain-protecting groups may first be cleaved by hydrogen fluoride (HF) treatment, and the peptide may subsequently be cleaved from the resin as the amide by ammonolysis. When the unsubstituted amide is desired, use of BHA or MBHA resin is preferred, because cleavage directly gives the amide.

The synthesis is conducted in a manner to stepwise add the amino acids in the chain in the manner set forth in detail in the U.S. Pat. No. 4,211,693, the disclosure of which is incorporated herein by reference. Side-chain protecting groups, as are well known in the art, are added to Ser, Tyr and His before these amino acids are coupled to the chain being built up upon the resin. A side-chain protecting group may also be added to Trp although oftentimes Trp is used without side-chain protection when solid phase synthesis is used. Likewise the α-amido group of pGlu may be protected or left unprotected.

Solid-phase synthesis provides the fully protected intermediate peptidoresin. The fully protected peptide can be cleaved, for example, from a chloromethylated resin support by ammonolysis to yield the fully protected amide intermediate. The intermediate of the invention may be generally represented as:

    (X.sup.1)pGlu--His(X.sup.2)--Trp(X.sup.3)--Ser(X.sup.4)--Tyr(X.sup.5)--R.sub.6 (X.sup.6)--Trp(X.sup.3)--Leu--Pro--R.sub.10 --X.sup.7

wherein:

X¹ is hydrogen or an α-amido protecting group of the type known to be useful in the art in the stepwise synthesis of polypeptides for the α-amido nitrogen of pGlu, such as one from the group (2) set forth hereinbefore, and preferably is Z when one is employed.

X² is a protecting group for the imidazole nitrogen of His and is selected from the group consisting of 2,4-dinitrophenyl, Boc, benzyl, Z and Tos; alternatively X² may be hydrogen, which means there is no protecting group on the side chain nitrogen atom.

X³ is hydrogen or a protecting group for the indole nitrogen, such as formyl or benzyl; however in many syntheses there is no need to protect Trp.

X⁴ is a protecting group for the alcoholic hydroxyl group of Ser and is selected from the group consisting of acetyl, benzoyl, tetrahydropyranyl, tert-butyl, trityl, benzyl and 2,6-dichlorobenzyl. Benzyl is preferred. Alternatively, X⁴ may be hydrogen.

X⁵ is a protecting group for the phenolic hydroxyl group of Tyr selected from the group consisting of tetrahydropyranyl, tert-butyl, trityl, benzyl, benzyloxycarbonyl, 4-bromobenzyloxycarbonyl and 2,6-dichlorobenzyl, the latter being preferred. Alternatively, X⁵ may be hydrogen.

X⁶ is hydrogen or a suitable protecting group for a labile side chain that is a part of the amino acid residue which appears as R₆.

X⁷ can be NH₂, or a protecting group for the C-terminal carboxyl group, such as an ester, substituted amide or hydrazide, or can be an anchoring bond to a solid resin such as: --O--CH₂ -resin support, --O--CH₂ -benzyl-polyamide resin support, --NH--BHA resin support, and --NH--MBHA resin support.

The criterion for selecting side chain protecting groups for X² -X⁶ is that the protecting group should be stable to the reagent under the reaction conditions selected for removing the α-amino protecting group at each step of the synthesis. The protecting group should not be split off under coupling conditions, and the protecting group should be removable upon completion of the synthesis of the desired amino acid sequence under reaction conditions that will not alter the peptide chain.

When the X⁷ group is --O--CH₂ -resin support or --O--CH₂ -benzyl-polyamide resin, the ester moiety of one of the many functional groups of the polystyrene resin support or of the polyamide resin is being represented. When the X⁷ group is --NH-resin support, an amide bond connects the C-terminal amino acid residue to a BHA resin or to an MBHA resin.

Use of the benzhydrylamine resin permits the C-terminal amide function to be obtained directly after the amino acid sequence in the synthesis is complete by cleaving from the resin support to form the glycine amide at the C-terminal portion of the peptide. When the other resins are used, the anchoring bond is the benzylester group which after cleavage of the peptide from the resin support must be converted to the C-terminal amide or substituted amide. The preferred procedure is to cleave the protected peptide from the resin support by aminolysis with an alkylamine or a fluoralkylamine for peptides with a Pro C-terminal or with NH₃ /MeOH or EtOH for peptides with a Gly C-terminal, and then to remove the protecting groups by hydrogenolysis or by hydrogen fluoride cleavage. An alternative procedure is to cleave by transesterification, e.g. with methanol/(Et)₃ N, and then convert the resulting ester into an amide and subsequently deprotect as described above (see Stewart "Solid-Phase Peptide Synthesis", pages 42-46). The latter procedure is preferably not used when R₆ has a side-chain carboxyl group protected by an ester.

In the Example set forth hereinafter, the α-amino protected amino acids are coupled step-wise in the desired order to obtain a compound of the desired formula; however, as an alternative to adding each amino acid separately to the reaction, some of them may be coupled prior to addition to the solid phase reaction.

After the desired amino acid sequence has been synthesized, the peptide may be removed from the resin support by treatment with a reagent, such as hydrogen fluoride, which not only cleaves the peptide from the resin but also cleaves all remaining side chain protecting groups and the α-amido protecting group (if present) on pyroglutamic acid to obtain the peptide directly in the case where the benzhydrylamine resin was used. Where a chloromethylated resin is used, the peptide may be separated from the resin by methanolysis after which the recovered product is chromatographed on silica gel and the collected fraction subjected to ammonolysis to convert the methyl ester to the C-terminal amide. Any side chain-protecting group may then be cleaved as previously described or by other procedures, such as catalytic reduction (e.g., Pd on C) using conditions which will keep the Trp moiety intact. When using hydrogen fluoride for cleaving, anisole is included in the reaction vessel to prevent the destruction of labile amino acid side chain (i.e., tryptophan).

Deprotection of the peptides as well as cleavage of the peptide from the benzhydrylamine resin preferably takes place at 0° C. with hydrofluoric acid (HF). Anisole is added to the peptide prior to treatment with HF. After removal of HF, under vacuum, the cleaved, deprotected peptide is treated with ether, decanted, taken in dilute acetic acid and lyophilized.

Purification of the peptide is effected by ion exchange chromatography on a CMC column, followed by partition chromatography using the elution system: n-butanol:0.1 N acetic acid (1:1 volume ratio) on a column packed with Sephadex G 25. The peptide of the invention is effective at a level of less than about 10 micrograms per 100 grams of body weight, when administered to fish. It is also considered to be effective in fish and in other vertebrates, including mammals and amphibians.

EXAMPLE

The following peptide having the formula

    pGlu--His--Trp--Ser--Tyr--D--Ala--Trp--Leu--Pro--NHCH.sub.2 CH.sub.3

is prepared by the solid phase procedure referred to above.

A chloromethylated resin is used, and the triethylammonium salt of Boc-protected Pro is esterified onto the resin over a 48-hour period by refluxing in ethanol. The proline residue attaches to the resin by an ester bond.

Following the coupling of each amino acid residue, washing, deblocking and coupling of the next amino acid residue is carried out in accordance with the following schedule using an automated machine for a synthesis begun with about 5 grams of resin:

    ______________________________________                                                                      MIX TIMES                                         STEP  REAGENTS AND OPERATIONS                                                                               MIN.                                              ______________________________________                                         1     CH.sub.2 Cl.sub.2 wash-80 ml. (2 times)                                                               3                                                 2     Methanol (MeOH) wash-30 ml. (2 times)                                                                 3                                                 3     CH.sub.2 Cl.sub.2 wash-80 ml. (3 times)                                                               3                                                 4     50 percent TFA plus 5 percent 1,2-eth-                                         anedithiol in CH.sub.2 CL.sub.2 -70 ml. (2 times)                                                     10                                                5     CH.sub.2 Cl.sub.3 wash-80 ml. (2 times)                                                               3                                                 6     TEA 12.5 percent in CH.sub.2 Cl.sub.2 -70 ml.                                  (2 times)              5                                                 7     MeOH wash-40 ml. (2 times)                                                                            2                                                 8     CH.sub.2 Cl.sub.2 wash-80 ml. (3 times)                                                               3                                                 9     Boc-amino acid (10 mmoles) in 30 ml. of                                        either DMF or CH.sub.2 Cl.sub.2, depending upon                                the solubility of the particular protected                                     amino acid, (1 time) plus DCC (10 mmoles)                                      in CH.sub.2 Cl.sub.2   30-300                                            10    MeOH wash-40 ml. (2 times)                                                                            3                                                 11    TEA 12.5 percent in CH.sub.2 Cl.sub.2 -70 ml.                                  (1 time)               3                                                 12    MeOH wash-30 ml. (2 times)                                                                            3                                                 13    CH.sub.2 Cl.sub.2 wash-80 ml. (2 times)                                                               3                                                 ______________________________________                                    

After step 13, an aliquot is taken for a ninhydrin test: if the test is negative, go back to step 1 for coupling of the next amino acid; if the test is positive or slightly positive, go back and repeat steps 9 through 13.

The above schedule is used for coupling of each of the amino acids of the peptide of the invention after the first amino acid has been attached. NαBoc protection is used for each of the remaining amino acids throughout the synthesis. The side chain of His is protected with Tos, but the side chain of Trp is not protected. OBzl is used as a side chain protecting group for the hydroxyl group of Ser, and 2-6 dichlorobenzyl is used as the side chain protecting group for the hydroxyl group of Tyr. pGlu protected by Z is introduced as the final amino acid. Boc-Trp has low solubility in CH₂ Cl₂ and is coupled using DMF.

Cleavage of the peptide from the resin is effected by treatment overnight with ethylamine at 0° C. Following recovery and washing, complete deprotection of the side chains of the intermediate is readily effected at 0° C. with HF. Anisole is added as a scavenger prior to HF treatment. After the removal of HF under vacuum, purification of the peptide is effected by semi-preparative reverse phase HPLC as described in Rivier et al. Peptides: Structure and Biological Function (1979) pp. 125-128. Purity is checked, and if the desired purity is not obtained, further purification is carried out by RP-HPLC using an acetonitrile gradient in 0.1% TFA.

The nonapeptide ethylamide is judged to be homogenous using thin layer chromatography and several different solvent systems, as well as by reversed phase, high-pressure liquid chromatography using an aqueous triethylammonium phosphate solution plus acetonitrile. Amino acid analysis of the resultant purified peptide is consistent with the formula for the prepared structure, showing substantially interger values for each amino acid chain. The optical rotation is measured on a photoelectric polarimeter and found to be [α]_(D) ²² =-31° (C=1, 1% acetic acid).

The peptide is assayed in vivo using salmon having an average weight of about 2 kg, and each fish is injected with about 10 μg. Additional fish are injected with the same amount of mammalian GnRH. Both sets are then compared with controls. About 2 hours following injection, the salmon are sacrificed, and the blood withdrawn for specific radioimmunoassay for LH level. The result of testing shows that the synthetic GnRH analog is substantially more potent than mammalian GnRH in causing the release of LH by the anterior pituitary in fish.

Additional nonapeptides and decapeptides having the compositions defined by the formula

    pGlu--His--Trp--Ser--Tyr--R.sub.6 --Trp--Leu--Pro--R.sub.10 --NHR

are synthesized in accordance with the procedure hereinbefore set forth as defined in the following Table:

                  TABLE                                                            ______________________________________                                         Peptide                                                                        No.         R.sub.6        R.sub.10 --NHR                                      ______________________________________                                         1.          D-Ala          NHCH.sub.2 CH.sub.3                                 2.          D-Lys          Gly--NH.sub.2                                       3.          D-Leu          NHCF.sub.2 CF.sub.3                                 4.          D-Orn          Gly--NH.sub.2                                       5.          D-Nal          Gly--NH.sub.2                                       6.          pCl--D-Phe     Gly--NH.sub.2                                       7.          imBzl--D-His   NHCH.sub.2 CH.sub.3                                 8.          O--tBu--D-Ser  NHCH.sub.2 CH.sub.3                                 9.          D-Trp          NHCH.sub.2 CH.sub.3                                 10.         D-Glu          Gly--NH.sub.2                                       11.         3Me--D-Val     Gly--NH.sub.2                                       12.         O--tBu--D-Asp  NHCH.sub.2 CH.sub.3                                 13.         OMe--D-Tyr     Gly--NH.sub.2                                       ______________________________________                                    

These peptides are considered to be more potent than mammalian GnRH in causing the release of LH by the test set forth above and thus useful in promoting the spawning of fish.

Based upon the foregoing, it is believed that these peptides can be used to regulate fertility in teleostei fish. More particularly, these peptides are expected to have substantial practical value in promoting spawning of teleostei fish of a wide variety of species, with immediate use being seen in fish hatcheries. In this respect, it is contemplated that administration may be by peritoneal or intramuscular injection to the fish at a dosage between about 5 μg. and about 100 μg. or less for fish having a body weight of about 100 grams. It is also felt that these peptides may be administered by adding to and preferably dissolving in water in which fish are caused to swim for a predetermined period during which the particular peptide will be absorbed into the bloodstream of the fish via the gills.

The frog has two GnRH-like peptides. In the adult frog brain, a peptide has been identified which is identical to mammalian GnRH; however, in the neonatal brain and in spinal ganglion, a different peptide is present, which it is now believed may have the same structure as Fish GnRH. It is also likely that the mammalian nervous system has a similar counterpart which is active within the central nervous system to modify sexual behavior. These synthetic peptides are active to stimulate gonadotropin secretion in a mammalian species, i.e. the rat, and it is possible that the peptides might be used to regulate gonadotropin secretion or sexual behavior in vertebrates other than fish, e.g. mammals, birds, reptiles and amphibians.

A peptide of this type is often administered in the form of a veterinarily or pharmaceutically acceptable, nontoxic salt, such as an acid addition salt or a metal complex, e.g., with zinc, iron, calcium, barium, magnesium, aluminum or the like (which is considered as an addition salt for purposes of this application). Illustrative of such acid addition salts are hydrochloride, hydrobromide, sulphate, phosphate, tannate, pamoate, oxalate, fumarate, gluconate, alginate, maleate, acetate, citrate, benzoate, succinate, malate, ascorbate, tartrate and the like.

These peptides or nontoxic salts thereof, combined with a pharmaceutically acceptable carrier to form a pharmaceutical composition, may also be administered to mammals, including humans, intravenously, subcutaneously, intramuscularly, percutaneously, intracerebrospinally or orally. For administration to humans, a peptide should be at least about 93% pure and preferably should have a purity of at least about 98%. This purity means that the intended peptide constitutes the stated weight % of all like peptides and peptide fragments present. Administration may be employed by a physician or by a veterinarian to regulate the secretion of gonadotropins and/or to cause ovulation. The required dosage will vary with the particular objective to be achieved and with the duration of desired treatment. Generally, dosages similar to those used with mammalian GnRH may be employed, and individual dosages between about 5 ng. and about 1 ug. per kilogram of body weight are used. At present time, the primary field of use is expected to be in teleostei fish where a peptide may be used at far lower purity, e.g. at about 5% purity or even as low as about 1% purity.

Although the invention has been described with regard to its preferred embodiment, it should be understood that changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.

Various features of the invention are emphasized in the claims which follow: 

What is claimed is:
 1. A peptide having the formula

    pGlu--His--Trp--Ser--Tyr--R.sub.6 --Trp--Leu--Pro--R.sub.10 --NHR

or a nontoxic salt thereof, wherein R₆ is a D-isomer of an α-amino acid selected from the group consisting of Trp, Ala, Phe, Lys, Pro, Met, Leu, Glu, Asn, Arg, Tyr, Cys, His, Chg, Nva, Orn, Thr, Abu, Phg, Ile, Glu, Asp, Nle and Val and wherein R₁₀ is Gly or des-R₁₀ and R is H or C_(n) Y_(2n) CY₃, with Y being H or F and n being 0, 1, 2 or 3, provided that when R₁₀ is des-R₁₀, n is not
 0. 2. A peptide in accordance with claim 1 wherein R₆ is D-Ala.
 3. A peptide in accordance with claim 1 wherein R₁₀ is des-R₁₀, Y is H and n is
 1. 4. A peptide in accordance with claim 2 wherein R₁₀ is des-R₁₀, Y is H and n is
 1. 5. A method of promoting the spawning of fish by administering an effective amount of

    pGlu--His--Trp--Ser--Tyr--R.sub.6 --Trp--Leu--Pro--R.sub.10 --NHR

or a nontoxic salt thereof wherein R₆ is a D-isomer of an α-amino acid selected from the group consisting of Trp, Ala, Phe, Lys, Pro, Met, Leu, Glu, Asn, Arg, Tyr, Cys, His, Chg, Nva, Orn, Thr, Abu, Phg, Ile, Glu, Asp, Nle and Val and wherein R₁₀ is Gly or des-R₁₀ and R is H or C_(n) Y_(2n) CY₃, with Y being H or F and n being 0, 1, 2 or 3, provided that when R₁₀ is des-R₁₀, n is not
 0. 6. A method in accordance with claim 5 wherein said administering is by injection.
 7. A method in accordance with claim 6 wherein a dosage of not greater than about 100 μg is administered for each 100 g. of fish body weight.
 8. A method in accordance with claim 5 wherein said administering is by dissolving in water wherein the fish are swimming. 